This invention relates generally to metallic oxide (i.e., ceramic) particles and to methods of their preparation and, more particularly, to substantially homogeneous, substantially spherical interdiffused metallic oxide particles and to methods of preparation of such particles from constituent metallic oxide powders.
Metallic oxide particles, typically comprising two or more metallic oxide powders, are useful in a variety of applications. A typical application is as a thermal barrier coating applied to gas turbine hot-gas-path metallic components. The thermal barrier coatings reduce the temperature of the metallic components, thereby increasing both the useful lives of the metallic components and turbine operating efficiencies. A typical method of applying thermal barrier coatings is to introduce the metallic oxide particles into plasma spraying apparatus which heats the particles to an elevated temperature sufficient to render the particles at least partially molten, propels the molten particles to the surface of the components to be coated, and causes the molten particles to impinge upon the component and rapidly cool thereby forming a solid ceramic thermal barrier coating over the component.
It is highly desirable that the metallic oxide particles for plasma spraying be substantially spherical in configuration in order that they flow freely through the plasma spraying device. Typically, preferred sizes for plasma sprayable particles are -200+325 mesh (44 to 74 .mu.m particle diameter). It is also highly desirable that the individual particles be as uniform as possible in composition in order to provide chemically homogeneous barrier coatings.
Metallic oxide particles may be formed from constituent metal oxide powders by several techniques. One such method involves the teachings of Krystyniak of U.S. Pat. No. 3,373,119, assigned to the same assignee as the present invention and incorporated herein by reference, wherein a slurry, made by adding a liquid vehicle and at least an organic binder to a dry blended mix of the constituent powdered metal oxides, is spray dried by equipment such as is shown in the U.S. Pat. No. 3,373,119, collected and sieved to final size. The product of this process is a mass of discrete agglomerated metal oxide particles substantially spherical in configuration and substantially uniform in composition. It has been found, however, that the discrete particles of agglomerated metal oxides produced by the Krystyniak process are substantially uniform in composition only on a macroscopic scale, i.e., there is generally little or no interdiffusion of the constituent metallic oxides such as to form a homogeneous solid solution, due to the relatively low temperatures at which the spray drying is conducted. In this art, it is also frequently common to refer to the resultant agglomerated particles of this and similar processes as " powders" in their own right.
Other means of preparing ceramic particles include (a) mixing the constituent metallic oxide powders together, pressing and binding the blend together to form an electrode, and arc melting the electrode to form a solidified mass which is then crushed, milled and sieved to yield the ceramic particles and (b) mixing the constituent metallic oxide powders together, pressing and binding the blend together to form briquettes, sintering the briquettes at an elevated temperature, and then crushing and milling the briquettes and sieving the product to yield ceramic particles in the desired size range.
The above-discussed (a) and (b) methods involve time consuming and expensive milling, crushing and sieving operations. Further, although the product of the above-discussed (a) and (b) methods is more physically and chemically homogeneous (i.e., interdiffused) than the particles produced by the above-discussed Krystyniak process, the vast majority of the particles produced as a result of the crushing process are angular in configuration as opposed to the free-flowing spherical configuration particularly desirable for use in plasma spraying applications. Adding to the expense of these methods is the low level of controllability which can be exercised over the yield of particles having sizes within a desired range such as -200+325 mesh.
Yet another means of preparing ceramic particles involves the process of dissolving soluble salts of the metals, such as the nitrates of the metals, in an aqueous media; coprecipitating the metals as oxalates or acetates from the aqueous media; and calcining the mixed products at about 1,000.degree. C. or greater to decompose the oxalates or acetates to form oxides of the metals. The chemically combined products of this expensive and involved process are generally characterized as "clumps", i.e., neither angular nor spherical in configuration, and generally are smaller than about 20-30 microns (.mu.m) which is generally smaller than the size range preferred for plasma spraying applications.
Thus, based on the above discussion, prior to the invention described herein, there existed a need for an inexpensive process for making substantially homogeneous, substantially spherical metallic oxide particles.